Add progressive web app companion for cross-platform access

Vite + TypeScript PWA that mirrors the Android app's core features:
- Pre-processed shelter data (build-time UTM33N→WGS84 conversion)
- Leaflet map with shelter markers, user location, and offline tiles
- Canvas compass arrow (ported from DirectionArrowView.kt)
- IndexedDB shelter cache with 7-day staleness check
- Service worker with CacheFirst tiles and precached app shell
- i18n for en, nb, nn (ported from Android strings.xml)
- iOS/Android compass handling with low-pass filter
- Respects user map interaction (no auto-snap on pan/zoom)
- Build revision cache-breaker for reliable SW updates

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pwa/tests/i18n.test.ts

@@ -0,0 +1,64 @@
+import { describe, it, expect, beforeEach, vi } from 'vitest';
+
+// Must mock navigator.languages before importing i18n
+const mockLanguages = { value: ['en'] };
+Object.defineProperty(globalThis.navigator, 'languages', {
+  get: () => mockLanguages.value,
+  configurable: true,
+});
+
+describe('i18n', () => {
+  beforeEach(async () => {
+    // Re-import to reset state
+    vi.resetModules();
+  });
+
+  it('returns English strings by default', async () => {
+    mockLanguages.value = ['en'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('app_name')).toBe('Tilfluktsrom');
+    expect(t('status_ready')).toBe('Ready');
+  });
+
+  it('detects Norwegian Bokmål', async () => {
+    mockLanguages.value = ['nb-NO'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('status_ready')).toBe('Klar');
+  });
+
+  it('detects "no" as Bokmål', async () => {
+    mockLanguages.value = ['no'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('status_ready')).toBe('Klar');
+  });
+
+  it('detects Nynorsk', async () => {
+    mockLanguages.value = ['nn'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('status_offline')).toBe('Fråkopla modus');
+  });
+
+  it('substitutes %d placeholders', async () => {
+    mockLanguages.value = ['en'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('status_shelters_loaded', 556)).toBe('556 shelters loaded');
+    expect(t('shelter_capacity', 200)).toBe('200 places');
+  });
+
+  it('falls back to English for unknown locale', async () => {
+    mockLanguages.value = ['fr'];
+    const { initLocale, t } = await import('../src/i18n/i18n');
+    initLocale();
+    expect(t('status_ready')).toBe('Ready');
+  });
+
+  it('returns key for unknown string key', async () => {
+    const { t } = await import('../src/i18n/i18n');
+    expect(t('nonexistent_key')).toBe('nonexistent_key');
+  });
+});